PP Duct Sizes: Diameter, Wall Thickness and Section Length

Written by Bofan Deng · Reviewed by XICHENG EP Engineering Department

Key Takeaways

• PP duct sizes are nominal outside diameters, not bores. A φ315 section with a 4.2 mm wall has a 306.6 mm bore, and the bore is what carries the air.
• PP duct sizes follow a nominal ladder rather than an arithmetic one: 110, 160, 200, 250, 315, 355, 400, 450, 500, 560, 630, 710, 800, up to 1200 mm. A size you need in between is a custom item, not a rounding error.
• An inch trade size is a label, not a dimension. An “8-inch” PP duct is a 200 mm product, not 203 mm, and a “3-inch” is a 90 mm product, not 76 mm.
• Wall thickness is selected for stiffness and buckling, not for internal pressure. In a vacuum exhaust system the external pressure is what collapses a panel.
• Rectangular and round ducts are compared with an equivalent diameter, and the equivalent round duct is always smaller than a round duct of the same area.
• A complete size line needs six items: diameter, wall, section length, joint type, pressure class and end preparation. Missing any one of them delays a quotation.

What PP Duct Sizes Actually Describe

A PP duct size is a statement about a manufactured part, not about a fluid passage. The number on the drawing and on the product page is the outside diameter of the finished section. That convention is not arbitrary: it is the dimension the tooling, the forming rolls, the welding jig and the flange all key off. It is the dimension a fabricator can actually hold.

The consequence is that the useful dimension and the stated dimension are different numbers. The air only sees the bore, and the bore is the outside diameter minus twice the wall. On a φ315 section with a 4.2 mm wall, the bore is 306.6 mm. Across the stocked series that gap runs from 6 mm at φ110 to 11 mm at φ500 — which does not sound like much until you notice that a 6 mm reduction on a 110 mm bore is 11% of the cross-sectional area, and therefore 11% of the airflow at the same velocity.

Nominal Diameter Is an Outside Diameter, Not a Bore

When a drawing says “φ315”, it means a section whose outside diameter measures 315 mm when it leaves the workshop. It does not mean a 315 mm bore, and it does not mean that a φ315 duct carries the same air as any other product also labelled 315.

This is the first place where two suppliers can quote the same nominal size and deliver two different systems. A supplier who prices to a nominal bore and then builds with thin walls ships a larger duct than one who prices to an outside diameter. The airflow calculation, the fan curve and the flange bolt circle all depend on which convention was used. The only safe practice is to state on the enquiry that the diameter is an outside diameter and to ask the supplier to confirm the wall at the same time. The convention is not universal across duct materials, either: FRP duct is commonly specified on an inside diameter with the wall built up around it, so the same quoted figure does not describe the same bore. That difference is set out in our comparison of PP duct and FRP duct.

For a fabricated, welded thermoplastic duct the outside-diameter convention is the one that survives contact with manufacturing. The outside is the surface the welding jig sees, the surface a flange ring is welded onto, and the surface that has to sit inside a support cradle. The inside is a weld bead field. Nobody can hold a tolerance on it.

Why the Ladder Is a Nominal Series, Not an Arithmetic One

If sizes were chosen by arithmetic progression the ladder would read 100, 150, 200, 250 and so on. It does not. It reads 110, 160, 200, 250, 315, 355, 400, 450, 500, and then jumps to 560, 630, 710 and 800.

The numbers recur across PP, PE and PVC pipe catalogues, because they descend from a metric outside-diameter series that predates any of these products’ popularity in ventilation. The reason the series is shaped this way is that each step is roughly a constant ratio rather than a constant increment. From 110 to 160 is a factor of 1.45; 200 to 250 is 1.25; 400 to 450 is 1.125; 630 to 710 is 1.127. A constant-ratio ladder means that somewhere in the range, whatever airflow you have, there is a size within about a quarter of a step of it — but only if you are willing to accept the discrete points. PVC pipe sizes come off the same outside-diameter series, which is why a size alone cannot tell the two materials apart; the differences sit in the chemistry, the joint method and the temperature ceiling, and they are set out in our comparison of PP duct and PVC duct.

The practical result is a set of small gaps that matter. Between φ500 and φ560 there is nothing. Between φ250 and φ315 there is nothing stocked, which is precisely where a great many branch and sub-main designs land once the velocity check is applied. The gaps are not an oversight to be worked around; they are the boundary of the standard range, and crossing them is a design decision with a cost attached.

The Three Numbers That Make a Complete Size

A diameter on its own is not a size. Three numbers together define a duct section: the outside diameter, the wall thickness, and the length of the section as supplied. Change any one and you have changed the part.

The wall thickness is the one buyers most often leave to the supplier, and it is the one with the largest downstream effect. Two φ400 sections from two suppliers can have walls of 4.5 mm and 6 mm. The heavier one is about a third more material, weighs about a third more, resists roughly 2.4 times the external pressure by the cube relationship that governs shell buckling, and holds a different support spacing. If the enquiry does not state a wall, the quotation is for whichever wall the supplier finds convenient, and the comparison between two quotes is meaningless.

The section length is the third number, and it is the one that most reliably surprises a first-time buyer. A 3 m section and a 6 m section contain the same duct, but they do not contain the same number of joints, and on a welded thermoplastic system the joints are where the labour is.

The PP Duct Size Ladder from 110 mm to 1200 mm

The gaps in the ladder are where the sizing risk lives, rather than where it is usually looked for. Because a 100 mm step on a 400 mm duct changes the bore by 25% and the velocity by about 36%, a velocity band narrower than the step can fall entirely inside one gap: a 5,800 m³/h pickling-mist duty lands at 13.42 m/s in φ400 and 8.58 m/s in φ500, one above the ceiling and one below the floor, so neither stocked neighbour works and the answer is a made-to-order diameter. How to spot that before a layout is frozen, and what the alternatives are, is set out in our guide to PP duct sizing.

The PP duct sizes that cover most industrial exhaust work run from φ110 mm to roughly φ1200 mm, in a stocked round series. Within that span the sizes are not evenly useful. Some are workhorses, some exist mainly to connect the ends of a range, and a few are effectively custom items that happen to have a standard name.

The Stocked Round Series and the Gaps Beside It

Reading the stocked PP duct sizes from the bottom: 110, 160, 200, 250, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1100, 1200 mm. Above 800 mm the available sizes thin out and the step between them widens, because fabricating a heavy-wall thermoplastic duct of a metre or more in diameter is a sheet-metal operation rather than a forming one, and the section is built to order.

The gaps are worth naming explicitly, because they are where designs stall:

• Between 250 and 315 — the single most common gap, and the one that catches branch and sub-main designs.
• Between 500 and 560 — a wide gap right where main ducts on mid-size systems want to sit.
• Between 800 and 900 — a 100 mm step that is a 12% jump in diameter and a 26% jump in area.

The 800-to-900 step is the largest in the range in area terms. Moving from φ800 to φ900 increases the cross-section from 0.503 m² to 0.636 m², which at a fixed airflow drops the velocity by 21%. That is a large, discrete change from a single step on the ladder, and it is why large-system designs often land on a custom intermediate rather than the next stocked size.

Which Band Carries Branches, Mains and Headers

Different parts of an exhaust system use different parts of the ladder, and the pattern is consistent enough to design against.

Branches and hood connections sit in the φ110 to φ200 band. A fume hood, a plating tank lip exhaust or a single pickling tank connection rarely needs more than a 200 mm duct, because the capture velocity at the hood face sets the flow, and the hood face is small. Below φ110 the sections are supplied in 4 m lengths rather than the 3 m used through the rest of the range, which reflects how these smaller sizes are made.

Sub-mains occupy φ250 to φ355. This is where two or three branches have combined and the velocity check starts to bite. It is also the band with the 250-to-315 gap in it, so a sub-main design that calculates to something in between has to choose between a smaller duct with higher velocity and a larger one with lower velocity and more material.

Mains and headers run from φ400 upward. A φ400 or φ500 duct typically forms the trunk of a multi-hood system feeding one scrubber or stack, and φ560 to φ800 appears where several mains combine before the air pollution control device. Above φ800 the duct is normally a short header or a scrubber inlet rather than a long run.

Sizes Below 110 mm and Above 800 mm

Below φ110 the ladder continues downward — 90, 75, 63, 50, 40, 32, 25 and 20 mm all exist as extruded PP duct — but the character of the product changes. These are laboratory and bench-scale sizes, and they are used for fume cupboard connections, small extraction arms and instrument vents rather than for plant exhaust.

Above φ800 the product is no longer a formed pipe at all. A metre-diameter PP duct is built from sheet: rolled or bent to a curve, closed with a longitudinal weld, ring-stiffened, and flanged at each end. The sizes above 800 mm are therefore best thought of as a set of convenient targets rather than a fixed series, and the practical upper bound of the range — around 1200 mm for a round duct — is set by handling and transport rather than by any manufacturing limit. A round section much beyond 1.2 m in diameter cannot be lifted or shipped economically, and the design moves to a rectangular profile instead.

PP Duct Sizes in Inches: Why 8-Inch Is Not 203 mm

Much of the industrial exhaust market specifies PP duct sizes in inches. The traps in that convention are worth understanding before a purchase order is written, because the inch figure and the metric product do not agree.

The Imperial Trade Sizes and the Metric Products Behind Them

In the North American market, PP duct is sold under trade sizes from 3 inch to 32 inch. Each trade size maps to a metric product, and the mapping is a convention rather than a conversion:

• 3 inch → 90 mm
• 4 inch → 110 mm
• 6 inch → 160 mm
• 8 inch → 200 mm
• 10 inch → 250 mm
• 12 inch → 315 mm
• 14 inch → 355 mm
• 16 inch → 400 mm
• 18 inch → 450 mm
• 20 inch → 500 mm
• 22 inch → 560 mm
• 24 inch → 630 mm
• 28 inch → 710 mm
• 32 inch → 800 mm

Compare the trade size with the arithmetic conversion and the scale of the mismatch appears. Three inches is 76.2 mm, but the product is 90 mm — 18% larger than its own name. Eight inches is 203.2 mm, but the product is 200 mm. Twelve inches is 304.8 mm, but the product is 315 mm, which is 3.3% larger. At 14 inch (355.6 mm against 355 mm), 22 inch (558.8 against 560) and 28 inch (711.2 against 710) the two nearly coincide, and those three sizes are the ones where a trade size and a metric size are effectively the same object.

The mismatch is not a defect in either system. It is what happens when a nominal inch label is applied to a product built on a metric outside-diameter ladder. The label describes the approximate size class; the metric figure describes the part.

A Metric-to-Inch Conversion Table

The pattern in the last column is worth pausing on. From 8 inch upward the metric product sits consistently about 1.6% below the arithmetic inch value, because the ladder was built on round metric numbers and the inch equivalents of those numbers are not round. In the small sizes the relationship inverts and the mismatch becomes large, because nobody sells a 76 mm duct when a 90 mm one is already on the shelf.

Why the Conversion Matters at the Flange

The difference between 200 mm and 203.2 mm is irrelevant to airflow — 1.6% of diameter is 3.2% of area, well inside any design margin. It is not irrelevant to a flange.

A flange is a rigid ring with a bolt circle, and it either matches the mating part or it does not. If a US-supplied fan or scrubber connection is drilled to a true 8-inch pattern and the duct arrives on a 200 mm outside diameter, the two do not bolt together, and no amount of sealant fixes a bolt-hole misalignment. The same applies to the reverse case: a duct fabricated to an inch pattern will not mate with a metric flanged valve.

The practical rule is to specify the mating face rather than the duct. State the bolt circle diameter and the hole count of whatever the duct has to connect to, and let the duct size follow from that. Where the duct has to mate with equipment from a different convention, the transition is made at a spool piece — a short, explicitly dimensioned section with the metric flange at one end and the inch pattern at the other.

Duct Wall Thickness by Diameter and Pressure Class

Wall thickness is the second number in a PP duct size, and it is the one that most often gets specified by habit rather than by calculation. The habit is usually a rule of thumb carried over from sheet-metal work, and for thermoplastic duct the governing load is different enough that the habit gives the wrong answer.

The Wall Is Chosen for Stiffness, Not for Pressure

An exhaust duct runs at close to atmospheric pressure. Even a large industrial system rarely exceeds a few thousand pascals of static pressure, which is a small fraction of one bar. Internal pressure is therefore almost never what decides the wall.

Two other loads decide it. The first is bending between supports: the duct’s own weight, plus any condensate or dust it carries, acting over the span between hangers. The second, and for exhaust systems usually the larger, is external pressure. A duct on the suction side of a fan is under vacuum, and a vacuum acts on the outside of the shell. A thin-walled tube under external pressure fails not by tearing but by buckling — it goes out of round and then flattens, at a pressure far below anything that would burst it from inside.

That is why negative pressure is the number that matters on an exhaust drawing. A vacuum that a thick panel shrugs off will collapse a thin one, and the collapse is sudden rather than gradual.

Negative-Pressure Classes and the Wall That Holds Them

The resistance of a cylindrical shell to external pressure scales with the cube of the wall-to-diameter ratio. Halving the ratio of wall to diameter cuts the buckling resistance by a factor of eight. The practical consequence is that the wall must climb with diameter to hold the same vacuum, and it must climb faster than proportionally.

The effect is visible in the stocked series. Our production range steps from a 3.0 mm wall at φ110 to a 5.5 mm wall at φ500. The diameter grows by a factor of 4.55 across that span, while the wall grows by only 1.83. By the cube relationship, the φ500 section has roughly one fifteenth the buckling resistance of the φ110 section at the same pressure differential, per unit of shell.

Two conclusions follow, and both are standard practice rather than rules of thumb. First, the pressure class belongs on the drawing, stated in pascals or millimetres of water gauge and marked as positive, negative or both. A fabricator cannot infer it from the diameter. Second, a large duct under significant vacuum is handled by stiffening rather than by piling on wall thickness, because the material cost of holding a vacuum with a heavier wall alone rises faster than the stiffness it buys.

Typical field numbers for sheet-built PP duct put a 5 mm wall at 2500 Pa of negative pressure for diameters under 600 mm, with larger or more highly loaded ducts moving to 6–8 mm or to external stiffening rings, and unreinforced positive pressure service limited to roughly 1500 Pa. Those are guidance figures for sizing a wall, not a substitute for a design check on the specific run. The per-diameter wall and mass figures for the stocked series, with the bore that each one actually delivers, are set out in the PP duct wall thickness and weight tables, where the same series is tabulated against its inside diameters.

When a Stiffener Ring Replaces a Heavier Wall

A stiffener ring works by shortening the unsupported length of shell. A long, unreinforced cylinder buckles at a pressure set by its wall-to-diameter ratio and its length; the longer it is, the lower that pressure. Welding a ring around the circumference divides one long shell into two short ones, and the buckling resistance of each short shell is substantially higher than that of the single long one it replaced.

The economics of that trade are straightforward. A ring of PP plate 20 mm thick and standing 50 mm proud, welded around a φ800 duct, adds a few kilograms of material and a few minutes of welding. Going from a 6 mm wall to a 10 mm wall over the same metre of duct adds far more material, weights the whole run, and loads every hanger. Above roughly 1000 mm in width or diameter, stiffening is the normal way to reach a pressure class, and a quotation that does not say whether the class is met by wall or by rings is an incomplete quotation.

The same logic applies in reverse to rectangular duct. A flat panel has no shell curvature to help it, so a square duct of a given wall is far weaker under vacuum than a round one. That is why square and rectangular PP duct is stiffened with transverse ribs at regular intervals rather than being built with a heavier wall throughout.

Round, Square and Rectangular PP Duct Sizes

PP duct sizes are not always best served by a round profile. Round duct is the default for industrial exhaust because it is the most efficient shape per unit of material, but it is not always the shape that fits. Where a duct has to run through a ceiling void or sit flat against a wall, a rectangular profile uses the available space better, and the sizing rules for it are different.

The Square and Rectangular Size Bands

Square and rectangular PP duct is a sheet-fabricated product, built to order rather than drawn from a stocked series. The practical range runs from about 200 × 200 mm up to 3000 × 1500 mm, with wall thickness stepping with the panel width:

Because the product is built from sheet, the size is genuinely free within the fabrication envelope — there is no mould and no dies to amortise. That is an advantage in a tight ceiling void and a disadvantage in a costing exercise, because two suppliers will not quote the same stiffening arrangement unless the enquiry specifies one. The panel grade is worth stating as well: PP sheet conforming to GB/T 12670 is the baseline, and a B1-grade flame-retardant sheet is available where the installation is fire-sensitive. The fabrication route for these sections is described on the square polypropylene duct page, including the dimensional tolerance held across panels.

Equivalent Diameter: Comparing a Rectangle to a Round Duct

A rectangular duct and a round duct cannot be compared on the basis of the stated dimensions, because they have different perimeters for the same area, and friction loss is driven by the wetted perimeter. The comparison is made through an equivalent diameter: the diameter of a round duct that produces the same friction loss as the rectangle at the same airflow.

The Engineering ToolBox equivalent diameter reference tabulates this relationship and gives the underlying expression, D_e = 1.30 (a·b)^0.625 / (a+b)^0.25, where a and b are the two rectangle sides. Working three examples through it:

• A 300 × 200 mm rectangle gives D_e = 267 mm.
• A 400 × 300 mm rectangle gives D_e = 378 mm.
• A 500 × 400 mm rectangle gives D_e = 489 mm.

The middle example is the instructive one. A 400 × 300 box behaves, in friction terms, like a 378 mm round duct — which is not a stocked size. The nearest round equivalents are φ355 and φ400, so an under-informed substitution either raises the velocity or adds material. Meanwhile the 500 × 400 case lands at 489 mm, within a whisker of a φ500 round duct, which makes that box a genuinely reasonable substitute for a round trunk in a space-constrained ceiling. Where the rectangle in question is a galvanized sheet-metal duct, that substitution is the usual route by which a PP run is proposed as a replacement, and the pair is compared in our article on PP duct vs galvanized duct.

Note also where the equivalent diameter falls relative to the size ladder. A 300 × 200 box needs a round equivalent of 267 mm, which sits in the 250-to-315 gap. This is the same gap flagged earlier, arrived at from a completely different direction, and it is a reminder that the gap is not an obscure corner of the range: it is where a great deal of ordinary design work lands.

Why a Round Duct of Equal Area Loses Less Pressure

For any rectangle, the equivalent diameter is smaller than the diameter of a round duct with the same cross-sectional area. The gap widens as the rectangle becomes more elongated.

Taking the same three shapes and comparing each with a round duct of equal area, a 300 × 300 square is about 3.2% below its equal-area circle, a 300 × 200 rectangle is 3.5% below, and a 400 × 200 rectangle is 4.4% below. The physical reason is perimeter. A 300 × 200 rectangle has 1000 mm of wetted perimeter; the round duct of equal area has about 868 mm. All of that extra wall is wetted surface, and every millimetre of it costs pressure.

That is the entire argument for round duct in one number, and it also quantifies when the argument does not apply. A 3–4% friction penalty is a real but modest cost to pay for solving a space problem. It is not a reason to reject square duct outright, and it is certainly not the 30–50% penalty that is sometimes implied by comparing a rectangle with a round duct of the same nominal dimension rather than the same area.

Section Length: 3 m, 4 m, 5 m or 6 m

The third number in a PP duct size is the length of the section as delivered. It gets the least attention in design and it has a direct effect on installed cost, because on a welded thermoplastic system every joint is field or shop labour.

Why the Standard Length Changes with Diameter

Our standard sections are 3 m through the main part of the range, with 4 m lengths supplied below φ110 where the product is a smaller extruded section that handles and stacks differently. Elsewhere in the market, 5 m and 6 m sections are also common, particularly for smaller diameters and for pipe-based products.

The length is not a free variable. It is bounded below by the cost of joints and bounded above by handling, transport and the physical stiffness of the section. A longer section means fewer welds per metre of run, which is why a fabricator would always prefer to ship longer. Against that, a long thermoplastic section is flexible, and a 6 m length of thin-wall duct sags under its own weight if it is not supported along its whole length during handling.

How Container Loading Sets the Export Length

For an exported system the upper bound on section length is a shipping container, and the arithmetic is unforgiving. A 20 ft general-purpose container gives roughly 5.9 m of internal length. A 6 m section does not travel in one. A 40 ft container gives roughly 12 m of internal length, which accommodates 5 m and 6 m sections with room to spare, but the same container loaded with 3 m sections carries twice the joint count per shipment.

The compromise that export projects usually reach is a 3 m or 4 m section in a 20 ft container. Three-metre sections pack efficiently, handle without a spreader, and can be manhandled down a plant corridor. Four-metre sections fit the same container with length to spare and reduce the joint count by a quarter. Above 4 m the sections start to demand lifting equipment on both the loading and the receiving end, and the saving in welds is paid back in rigging.

There is a second transport constraint that applies to large diameters regardless of length. A φ1000 round section is 1 m across, so only a limited number fit side by side in a container before the width is used up. On large-diameter projects the shipped length is often chosen to suit the stacking pattern rather than the weld count, and the enquiry should say so.

Joint Count, and What the Joints Cost

The number of joints in a run is simply the run length divided by the section length, rounded up, plus one. On a 40 m straight run the difference is stark: 3 m sections give 14 joints, 4 m sections give 11, and 5 m sections give 9.

Each joint on a welded PP duct is a flange pair or a butt weld with a gasket, a set of fasteners, and a welder’s time. The material in a flange pair is not trivial either: on a φ315 duct, a pair of PP flange rings 20 mm thick and 1.5 times the duct diameter adds around 3.5 kg to the run for every joint. On the 40 m run, moving from 3 m to 5 m sections removes five flange pairs and about 17 kg of flange material, plus the fasteners and the welding time for five joints.

The counter-argument is that a longer section is harder to install in a congested plant room, and that a joint placed at the wrong point in a run is worse than an extra joint placed at a convenient one. Section length is a design decision, not a purchasing convenience, and it belongs on the enquiry alongside the diameter and the wall. How the chosen length then feeds into support spacing, hanger loads and the position of the fixed points is worked through in our guide to PP duct installation.

Diameter Tolerance and What to Accept

Wall thickness and tolerance interact differently once the duct is specified in a flame-retardant grade, because that compound carries a tensile strength roughly 10 to 15% below the standard homopolymer. The wall needed to hold the same duty therefore rises, and a nominal diameter held at a fixed outside dimension will deliver a slightly narrower bore at the thicker wall — which moves the velocity, and with it the friction figure the fan was sized against. Checking the bore rather than only the outside diameter is the habit that catches it. How the grade changes the numbers is set out in our guide to a flame retardant PP duct.

PP duct sizes are nominal figures, and the tolerances around them reflect the fact that a fabricated thermoplastic duct is not a machined part. Knowing what a reasonable tolerance is prevents both the rejection of acceptable goods and the acceptance of a section that will not align on site.

Dimensional Tolerance on a Fabricated Section

On sheet-fabricated square and rectangular duct, a dimensional tolerance of about ±2 mm across the panels is the working figure, held by the cutting and bending process. On a formed round section the relevant figure is the outside diameter at the ends, since that is what the flange keys off and what has to meet the mating part.

The dimension that matters for assembly is not the roundness of the middle of the section but the geometry of the two ends. A section that is 3 mm out of round in the middle but has two true ends will bolt up perfectly. A section with a true barrel and a distorted end will not, and it will not be obvious until the flange is offered up.

Roundness and Wall-Thickness Variation

Two further measurements determine whether a section is fit for purpose.

Roundness matters most where the duct has to pass through a support cradle, a wall penetration or a sleeve. A formed section that has been stored on its side under load will take a set, and a section that was round when it left the workshop may not be round when it arrives. The practical acceptance criterion is that the section can be rotated by hand to bring the bolt holes into alignment — if it cannot, the flange will not seat.

Wall-thickness variation matters because the wall is what resists buckling, and a section that is nominally 5.5 mm but runs 4.8 mm at one point has a lower collapse pressure than its nominal figure suggests. Because buckling scales with the cube of the wall, a 13% local reduction in wall is a 34% reduction in the local buckling resistance. That is a large enough effect to be worth a measurement, and a simple ultrasonic thickness gauge on a handful of points around each end of the first section delivered is enough to confirm that the ordered wall is being supplied.

What to Reject at Goods-In

A short goods-in inspection catches the failures that matter, and it does not need to be elaborate. Four checks cover most of it:

• Measure the outside diameter at both ends of one section per batch. The two ends should agree with each other and with the ordered figure.
• Measure the wall at several points around the circumference at each end. Look for the minimum, not the average.
• Check the ends are true and that the flange faces are square to the axis. A flange that is out of square will not pull up, and forcing it distorts the duct.
• Look at the weld around the full circumference at each end, and at the longitudinal seam. A pinhole in a weld will not show up in a dimensional check and will show up later as a leak or as corrosion starting at the bead.

The one thing not worth rejecting is a small deviation in the middle of a long section. A section that is straight enough to install and true at both ends will perform to its nominal bore regardless of what the barrel does in between.

PP Duct Sizes Are Not PP Pipe Sizes

A search for PP duct sizes returns a large number of results that are about polypropylene pressure pipe. The two products share a material and a family of nominal outside diameters, and almost nothing else. Confusing them is the single most common error in specifying a PP system, and it leads to drawings that no fabricator can build.

DN, PN and SDR Belong to Pressure Pipe

Pressure pipe is specified by three pieces of vocabulary that do not transfer to duct.

DN is a nominal diameter, a dimensionless size label, and on many plastic pipe systems it corresponds to the outside diameter. It tells you the size class, not the bore.

PN is a nominal pressure rating — PN 10 is 10 bar. It describes the internal pressure the pipe will contain, and it is set by the wall thickness in relation to the diameter.

SDR is the standard dimension ratio, the outside diameter divided by the wall thickness. A lower SDR means a heavier wall and a higher pressure rating. SDR 11 and SDR 17 are common polypropylene pressure pipe ratios.

None of these three concepts describes a ventilation duct. A PP duct is not rated in bar; there is no internal pressure of that magnitude anywhere in an exhaust system. It is not specified by a dimension ratio, because the wall of a duct is chosen for buckling and for support spacing rather than for a pressure containment relationship. And its nominal size, while an outside diameter, is a manufacturing series rather than a DN class.

Why a Schedule Number Does Not Apply to Welded Duct

In the metal world, a schedule number performs the same job as SDR: it ties a wall thickness to a diameter to give a pressure rating. Schedules are a useful shorthand precisely because metal pipe is made in a standard mill series where that relationship is fixed.

A welded thermoplastic duct is not made that way. The wall is a sheet thickness selected from what is available, welded to a shell of whatever diameter the design needs, and stiffened as required. Two φ400 PP ducts with the same wall can have completely different vacuum ratings depending on their length, their stiffener spacing and their support arrangement. There is no single wall-to-diameter table that fixes the rating, and therefore no schedule equivalent.

This is why a PP duct specification has to state the pressure class directly rather than pointing at a schedule. It is also why a quotation for “schedule 40 PP duct” is a sign that the supplier has not understood the product.

The Standards Each Product Answers To

The two products are governed by different standards, which is the clearest test of which one is on the drawing.

Polypropylene pressure pipe is covered by standards such as ASTM F2389 for pressure-rated polypropylene piping systems, and by the ISO 15874 series for hot and cold water installations. These standards are concerned with pressure containment, hydrostatic design stress and long-term creep rupture — the concerns of a pipe carrying fluid under pressure.

Polypropylene ventilation duct is covered by standards concerned with fabrication, welding and structural performance. Material for the sheet and moulded components is specified against ASTM D4101 for polypropylene materials, or against GB/T 12670 for PP sheet. Welding is performed to the DVS 2207 series — 2207-3 for hot-gas welding and 2207-4 for extrusion welding — and the structural approach follows the thermoplastic duct construction manuals, of which the SMACNA thermoplastics manual is the reference most often cited on drawings.

If a specification cites PN or SDR, it is describing a pipe. If it cites DVS 2207 or a pressure class in pascals of vacuum, it is describing a duct. The two vocabularies do not mix, and a drawing that contains both is a drawing that has not been resolved.

From a Stocked Size to a Written Size Line

Everything above resolves into a practical sequence: read PP duct sizes off the ladder, check the chosen one against the duty, and write it down in a form a fabricator can quote from.

Reduce to a Stocked Size Before You Go Custom

The first move is almost always to check whether a stocked size will do. A size taken off the ladder is available as a standard product, has an established wall and section length, and carries no tooling or drawing cost. A size from between the rungs is a custom item, which means a drawing, a lead time and a minimum quantity.

The check is a velocity check, and it is deliberately loose. Take the design airflow, divide by the cross-sectional area of the candidate bore, and see whether the velocity lands inside the band that suits the duty. For corrosive fume exhaust that band is roughly 12–18 m/s — high enough to prevent condensation and keep dust moving, low enough to avoid excessive fan energy and noise. For dust-laden air the floor is higher, around 16–22 m/s, because the minimum transport velocity has to exceed the settling velocity of the particles.

If a stocked size puts the velocity anywhere inside the band, that size is correct and the design should stop there. The full treatment of airflow, velocity and pressure loss belongs to a separate design exercise; what matters for a size line is that the stocked size passes the check. The worked airflow-and-velocity method, and the relationship between velocity, diameter and fan power, is developed in the polypropylene duct design guide.

Only when no stocked size passes the check does the design move to a custom diameter. The two legitimate reasons to do so are a velocity outside the band on both the size below and the size above, or a physical constraint — a maximum height in a ceiling void, or a mating dimension on existing equipment.

A Specification String a Fabricator Can Quote From

A PP duct size line that a fabricator can quote from without asking a question looks like this:

φ315 OD × 4.2 mm wall × 3000 mm, PP-H, hot-gas welded to DVS 2207-3, flange both ends to a 12-hole bolt circle, 2000 Pa negative, 16 m/s design velocity, total run 24 m with 3 × 90° bends.

Every element in that string answers a question the fabricator would otherwise have to ask. The diameter is marked as an outside diameter, so there is no ambiguity about the bore. The wall is stated, so the quotation can be compared with another one. The section length is stated, so the joint count and the flange quantity can be priced. The material grade is stated, because PP-H, PP-B and PP-R behave differently at temperature. The welding method is stated, because it determines the joint preparation. The end preparation is stated, because a flanged end and a plain end are different operations. The pressure class is stated, because it decides whether the wall is adequate or a stiffener ring is needed. And the total run length with the fitting count is stated, because it tells the fabricator how many sections and how many joints are involved.

The Omissions That Delay a Quotation

In practice, five omissions account for most of the back-and-forth on a PP duct enquiry.

The wall thickness is left out more often than any other figure. Without it, a quote can only be priced against a default, and two quotes against two different defaults are not comparable.

The pressure class, and specifically whether it is positive, negative or both, is the second. An exhaust duct is almost always in vacuum, and the vacuum figure is what decides the wall. A quote priced without it is priced against an assumption.

The end preparation is the third. Flanged, plain, and socket ends are different fabrication operations with different material content, and a mix of them across a run needs to be itemised rather than averaged.

The section length is the fourth, and it is the one most often assumed. If the enquiry does not state it, the supplier will choose, and the choice will favour the supplier’s handling rather than the site’s access.

The mating dimensions are the fifth. Whatever the duct has to bolt to — a fan inlet, a scrubber nozzle, an existing spool — needs its bolt circle and hole count on the drawing. A duct that fits the run but not the equipment is not a useful duct.

Sizes That Need a Custom Quote

A meaningful share of industrial work falls outside the stocked ladder, and knowing which categories do saves a wasted enquiry against the standard range.

Intermediate Diameters and Off-Series Sizes

Any diameter between the rungs of the ladder is a custom item. The commonest are the ones that fill the wide gaps: φ280 between 250 and 315, φ450 between 400 and 500 (though this one is stocked on some ranges), φ530 between 500 and 560, and anything between 800 and 900.

A custom diameter is normally built from sheet to the same construction as the sizes above 800 mm, since there is no forming tool for a one-off size. That makes the cost step larger than the diameter step suggests. Where the design can be made to work on a stocked size with a slightly different velocity, that is almost always the cheaper answer.

Large Square and Rectangular Sections

Square and rectangular duct above the standard wall bands, and particularly above 2000 mm in one dimension, moves into a different construction class. Panels at 8–12 mm are heavy, the stiffening arrangement becomes a structural design rather than a detail, and corner gussets and longitudinal stiffeners start to appear. These sections are quoted individually against a drawing.

The same applies to long rectangular runs at moderate width. A 1500 × 400 mm duct is not a large section in area terms, but it is an awkward one structurally, and its stiffening schedule has to be designed rather than selected.

Transitions Between Two Sizes

Any change of size within a run needs a transition piece, and transitions are fabricated items rather than stocked ones. A reducer between two round sizes is a straightforward welded cone; one between a round duct and a rectangular duct is a square-to-round transition, which is a developed-surface fabrication with its own sheet-metal layout.

Two sizing rules govern these pieces. A transition should be gradual: the established practice is a length of about 1.5 times the larger diameter for a square-to-round, and a similar proportional length for a round reducer, because a short, abrupt transition generates turbulence and pressure loss out of proportion to its size. And the transition should be placed where a change of velocity is acceptable, which usually means away from the fan inlet and away from the point where a branch joins.

The behaviour of transitions and other fittings in a system — the loss coefficients, the effect of curvature ratio, and the difference between a long-radius and a mitred bend — is treated in the PP duct fittings guide. What matters for sizing is that a transition is a line item with a length, not a detail that can be left off the drawing and absorbed into the run.

Frequently Asked Questions

What sizes does PP duct come in?

Round PP duct sizes run from about φ20 mm up to around φ1200 mm. The stocked industrial range that carries most exhaust work runs from φ110 mm to φ800 mm in a nominal series — 110, 160, 200, 250, 315, 355, 400, 450, 500, 560, 630, 710 and 800 mm — with larger sizes to 1200 mm and beyond available as built-to-order sheet sections. Square and rectangular PP duct is fabricated to order from about 200 × 200 mm up to 3000 × 1500 mm.

Is PP duct measured by inside or outside diameter?

By outside diameter. A size stated as φ315 is a section whose outside diameter measures 315 mm, and its bore is smaller by twice the wall thickness. With a 4.2 mm wall the bore is 306.6 mm. Because the outside diameter is the dimension that tooling, flanges and supports key off, it is the one that can be held to tolerance, and it is therefore the one that appears on the drawing.

What is the most common PP duct size?

For industrial exhaust the most frequently specified sizes are φ200 mm on branches and hood connections, φ315 mm on sub-mains, and φ400 mm and φ500 mm on mains feeding a scrubber or stack. Small laboratory and fume cupboard work sits lower, in the φ110 to φ160 band. The distribution reflects the two constraints that drive most designs: the capture velocity needed at the hood, which sets the branch size, and the transport velocity needed in the duct, which sets the main.

What wall thickness do I need for a 315 mm duct?

A φ315 mm PP duct in the stocked series is supplied with a 4.2 mm wall and a resulting bore of 306.6 mm, which suits the ordinary range of exhaust duties up to a few thousand pascals of negative pressure. Where a φ315 duct has to hold a higher vacuum, or has to span further between supports than the standard spacing allows, the wall moves up and the duct becomes a custom item. The governing loads are external pressure and bending between supports, not internal pressure.

Can I get a PP duct in an inch size?

Yes, but the inch figure is a trade label rather than a dimension. A North American “8-inch” PP duct is a 200 mm outside-diameter product, not a 203.2 mm one, and a “3-inch” is a 90 mm product rather than a 76.2 mm one. The mismatch runs from about 18% at the small end to under 2% from 8 inch upward, and it only becomes a practical problem where the duct has to mate with equipment drilled to a genuine inch bolt pattern. In that case, specify the mating bolt circle rather than the duct size, and resolve the difference in a short transition spool.

What is the largest PP duct size?

Round PP duct is made up to roughly φ1200 mm as a built-to-order sheet section, beyond which handling and transport make the round profile impractical and the design switches to a rectangular duct. Rectangular PP duct is fabricated up to about 3000 × 1500 mm. Both are custom products above the stocked range, built to a drawing with stiffening designed for the pressure class rather than selected from a table.